The rancor that burst into the open on Friday marked a sharp shift in a Democratic primary that has been devoid of drama, particularly compared to the unpredictable Republican contest. The controversy appeared likely to overshadow any policy discussions in Saturday's prime-time debate in New Hampshire, the party's third contest and the last of the year.

Clinton and Sanders will be joined on stage by former Maryland Gov. Martin O'Malley, who has struggled to become a factor in the race. The debate is expected to draw low viewership, given that it's scheduled on the last weekend before Christmas, when most Americans have turned their attention to the holiday season.

For Clinton, the question was how forcefully to confront Sanders about the data breach and whether to defend the reaction of the Democratic National Committee, which temporarily cut off the Vermont senator's access to the party's voter database. Sanders' campaign said its access was restored early Saturday morning.

Sanders vowed to avoid personal attacks during the campaign and gave Clinton a pass on her use of a private email account and server while serving as secretary of state in the first Democratic debate. But his campaign immediately leapt on the DNC actions in the data dust-up, sending a fundraising email to supporters that said the national party had placed "its thumb on the scales in support of Hillary Clinton's campaign."

The DNC maintains a trove of voter information. The campaigns can add to that database — information they use to target voters and anticipate what issues might motivate them.

In Clinton's case, campaign manager Robby Mook said that information included "fundamental parts of our strategy." Clinton aides said the information that the four Sanders workers reviewed in 25 separate searches included details on voter turnout and candidate preferences, revealing the Clinton campaign's approach in early voting states such as Iowa and New Hampshire.

Experts said the Sanders campaign employees who accessed it without authorization appear to have broken the law.

"Our data was stolen," Mook said. "The data that they reached in and took from our campaign is effectively the strategic road map in those states."

Sanders' campaign fired a worker involved in the data breach and campaign manager Jeff Weaver admitted that the worker's actions were "unacceptable." But the campaign rejected the allegations the Sanders team stole data and sued the DNC to regain access to the voter records. The suit contended the DNC's actions caused Sanders' campaign "injury and financial losses."

The issue never came to court, as the DNC said early Saturday morning the Sanders campaign had complied with its request for information about the incident.

"Based on this information, we are restoring the Sanders campaign's access to the voter file, but will continue to investigate to ensure that the data that was inappropriately accessed has been deleted and is no longer in possession of the Sanders campaign," DNC Chairwoman Debbie Wasserman Schultz, a Florida congresswoman, said in a statement.

She later told CNN that the Sanders' campaign was "cooperating fully" with the party's investigation. "It's time now to turn to the issues that matter to Americans, because it's certainly in stark contrast to the extremism that we're seeing on the Republican side."

While Clinton's campaign launched to a shaky start following revelations about her private email use at the State Department, she rebounded in the fall and now has a commanding lead of 20 points or more in most national polls. Yet if Clinton is aggressive in taking on Sanders over the data breach, she could risk alienating his passionate liberal voters, whom she would need to win in a general election.

An independent running as a Democrat, Sanders has deeply loyal supporters who are drawn to his economic- and inequality-focused campaign. However, he's struggled to maintain momentum since national security and terrorism concerns shot to the front of the White House race after attacks in Paris and San Bernardino, California.

"He's got to refocus Democrats onto his issue ground," said Paul Maslin, a Democratic pollster unaffiliated with either campaign. "It's been usurped by events."

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